1. Field of the Invention
The present invention relates generally to an optical network system configured by connecting a plurality of systems through an optical fiber network, and more particularly to an optical network system for bidirectional transmission between a main system (e.g., central station) and a plurality of auxiliary systems (e.g., subscriber's stations).
2. Description of the Related Art
An example of a conventional optical network system will now be described with reference to FIG. 1. This conventional system includes a main system 100 for outputting a main optical signal to a main fiber 26, a plurality of auxiliary systems 200 (#1 to #N) respectively connected to a plurality of auxiliary fibers 36 (#1 to #N), for outputting auxiliary optical signals to the auxiliary fibers 36 (#1 to #N), respectively, and a branching unit 300 for network-connecting the main fiber 26 and the auxiliary fibers 36 (#1 to #N).
In the case of providing the service of VOD (Video On Demand) by this optical network system, the main system 100 is a central station connected to a provider of a video signal, and each of the auxiliary systems 200 (#1 to #N) is a subscriber's station.
Assuming that a plurality of main fibers are present as the main fiber 26, a star configuration is provided with respect to the main system 100. Further, as the plural auxiliary fibers 36 (#1 to #N) are connected to the branching unit 300, a star configuration is provided also with respect to the branching unit 300. In this case, the optical network system is called a double star. Particularly in the case that the branching unit 300 includes only passive components such as an optical star coupler, the above optical network system is called a passive double star (PDS). The detail of this conventional system will be hereinafter described in contrast with the present invention.
In recent years, an optical amplifier such as an EDFA (erbium doped fiber amplifier) has been developed. In the above optical network system, for example, the main optical signal can be distributed to the plural auxiliary optical fibers 36 by amplifying the main optical signal. When such a high-power optical signal is supplied to the branching unit 300, the optical power in each auxiliary fiber 36 also becomes high, causing a possibility of laser hazard. For example, if any of the auxiliary fibers 36 is broken, it is required to cope with laser hazard, so as to repair a break point in this broken auxiliary fiber. Further, when an optical connector connected to any of the auxiliary fibers 36, there is a possibility that a high-power optical signal is radiated to a general user from the connector end. Therefore, it is also required to cope with this radiation.